This invention relates to the detection of ammonia or to the detection of the ill effects of ammonia in flue gas.
Ammonia is in common use today as a reactant for the removal of nitrogen oxides from gas streams. When it is injected it reacts with nitric oxide (NO) to form N2 and H2O and thereby reduces the emissions of the undesirable nitrogen oxides. It is usually used in concentrations about as large as the NO concentration.
Two common methods are used to speed the reactions between ammonia NH3 and NO. In one case a high temperature is used. Temperatures of about 1600xc2x0 F. to 1900xc2x0 F. are used to speed the reaction. After this reaction the gases, if they are from combustion in a boiler, pass through several heat exchange devices and they eventually exit the stack at about 270xc2x0 F. to 370xc2x0 F. The gases from some older boilers may exit the stack at higher temperatures, but for efficiency it is necessary to have low stack temperatures. This type of process is known as Thermal deNOx or Selective Non-Catalytic Reduction (SNCR). In another case a catalyst is used to speed the reaction. Even so the catalyst bed may be at around 700xc2x0 F. This process is known as Selective Catalytic Reduction (SCR). Subsequent to the reduction, the gas is cooled to the same temperatures as in the Thermal deNOx systems.
In either of these processes some of the NH3 passes through the reaction zone and out of the stack along with the flue gas. It is undesirable to have NH3 in the flue gas as it is seen as an undesirable emission and in many places there are regulations limiting the NH3 emissions. The odor of ammonia is objectionable. When the ammonia is too high some will be absorbed by the fly ash. The ash then has the odor of ammonia and must be disposed of rather than used in concrete. This adds an expense to the operation of the boiler.
NH3 also reacts with chlorine or hydrochloric acid, either of which may be in flue gas from the combustion of coal and are usually in the combustion of refuse and some wood waste. The reaction forms ammonium chloride (NH4Cl). NH4Cl forms as a very fine particle or fume, which makes an objectionable visible emission. Also, the NH4Cl can plug various heat exchange devices and stop the flow of the flue gas.
Most importantly the NH3 reacts with sulfur trioxide (SO3) to form ammonium sulfate ((NH4)2SO4) or ammonium bisulfate (NH4)HSO4. They both can plug heat transfer devices especially the regenerative air heater, which has many small passages. This plugging can restrict the flow of the air and the flue gas so severely that the boiler must be taken off-line and the air heater cleaned. The ammonium bisulfate is much the worst offender of the two as it is very sticky through much of the exhaust gas temperature range. The problem may be exacerbated by the SCR process that converts some of the SO2 to SO3. Also, the ammonia may react with SO2 and oxygen to form these ammonium sulfates. Since the SO2 is much more abundant (about 50 to 1) than the SO3 all of the ammonia present in flue gas is likely to react and form an ammonium sulfate.
By whatever process the ammonia reacts or how ammonium sulfates are formed, all of the ammonium salts that are present in the flue gas can cause fouling of heat exchanger surfaces and plugging of the heat exchangers. This shuts down the operation. Ammonium bisulfate is the worst offender since it melts at 296xc2x0 F. Ammonium sulfate is solid to 955xc2x0 F. where it decomposes and ammonium chloride is solid to 662xc2x0 F. where it decomposes. Thus, while any of these salts can cause fouling, only the ammonium bisulfate will exist in the liquid state in the boiler and the liquid is the source of the greatest fouling problem.
The fouling of heat transfer surfaces by liquid ammonium bisulfate and the solid particles that are imbedded in the liquid can become very severe at temperatures above 296xc2x0 F. The ammonium bisulfate decomposes at high temperatures, maybe as high as 914xc2x0 F. and the fouling problem could extend to temperatures this high.
Ammonium bisulfate, which is often called ammonium acid sulfate is acidic and can cause corrosion especially in the presence of water. Thus, the molten ammonium bisulfate may cause a water dew-point at temperatures significantly above its melting point of 296xc2x0 F. In that case, the water present in the flue gas is induced, by the ammonium bisulfate, to condense at higher and higher temperatures where it should not normally condense. This water dew point further aggravates the fouling tendencies of the ammonium bisulfate in that it allows for the condensation of a sticky water-soluble material which in turn causes fly ash to also accumulate (or foul) at temperatures above 296xc2x0 F. How far above this temperature that fouling occurs is a measure of the fouling tendency caused by an excess of ammonia.
To eliminate the plugging it is necessary to eliminate the formation of ammonium sulfates in the flue gas. The sulfur as well as chlorine are in the fuel whether it is coal, oil, waste or other combustible material. Consequently, to prevent heat exchanger plugging as well as to reduce emissions of ammonia and prevent this source of corrosion it is necessary to reduce the amount of ammonia (i.e., ammonia slip) that passes through the SNCR or SCR process. To do this, it is very important to be able to measure either the ammonia slip or the resulting fouling tendency. Typically, ammonia is introduced into flue gas at multiple locations around the circumference of the stack to reduce NOx. The problem with reducing the ammonia is that it is essential for the NOx reduction processes. Because ammonia is introduced at multiple locations and because of turbulence and cross currents in the flue gas, the concentration of ammonia may be too high in one location and not high enough in another part of the process. A measurement device is needed to find the ammonia concentrations on a spatial basis and in real time. Instruments that are available for the measurement of ammonia have not been reliable and wet chemical analysis of the gas for ammonia is too slow for control purposes. If there were a reliable instrument and method for measuring ammonia slip on a spatial basis in real time, the ammonia slip measurement could then be used to optimize the spatial injection of ammonia into any combination of SNCR and SCR processes.
We provide a method of measuring ammonia in flue gas by using a cooled probe to measure conductivity (and corrosion) caused by condensed ammonium bisulfate. The process will work for any fuel with a significant concentration of sulfur, i.e., where there is potential for the fouling and corrosion problem to occur. The method and probe may also reveal those circumstances where there is no problem in fouling or corrosion occurring. It will be most useful in furnaces or boilers where the operator is injecting ammonia to reduce NOx emissions. The use of this method and device will allow the boiler operator to use as much ammonia as is required to substantially eliminate NO emissions without fear of fouling the boiler back passes and without excessive ammonia emissions.
We prefer to provide a tubular probe having spaced apart bands or patches of the same material as the probe body. The bands or patches are attached to the probe body by an electrically insulating, high temperature material. At least one thermocouple is attached to the probe. A series of cooling tubes are provided within the probe body to direct cold air to the regions near each band. One or more probes is placed in the furnace or boiler above the ammonia injection zone. When ammonium bisulfate forms on the probe it completes an electrical circuit between the probe body and the bands. Hence, the presence of ammonium bisulfate can be detected by a change in resistance between the bands and the probe body. The ammonium bisulfate will also cause corrosion of the probe. Electrochemical noise is generated during the corrosion process. A monitor connected to the probe body can detect any change in resistance as well as electrochemical noise. Furthermore, a corrosion rate can be determined from the level or amount of electrochemical noise that is detected.
Information obtained from the probe can be correlated with the position of the probe to identify those injectors that may be the source of the detected excess ammonia. Then the injectors can be adjusted to reduce or eliminate excess ammonia injection.